Butyl rubbers, which are copolymers of isobutylene and isoprene, have been around since the 1930s. Butyl is a crosslinkable thermoset rubber with majority polyisobutylene (PIB) so to have the excellent damping and exceptional impermeable properties of PIB. The commercial process to make butyl rubber involves cationic copolymerization of isobutylene and isoprene in methyl chloride diluent using aluminum chloride initiating system.
Isoprene, a conjugated diene, was added to butyl rubber as a co-monomer to make the resulting rubber crosslinkable. Isobutylene is perfectly suited for cationic polymerization with the formation of tert-butyl cations after its protonation, but isoprene is not. Therefore, the isoprene content within a butyl rubber is limited, if it is incorporated by cationic copolymerization using an aluminum chloride initiator. The isoprene content is typically below 5 mole %, and mostly below 3 mole %. Moreover, the double bond of the resulting polymer is in the backbone, so its accessibility is restricted leading to slow curing (or crosslinking) kinetics.
Despite excellent impermeability, butyl rubber lacks polarity, which renders it highly immiscible, incompatible, and non-adherent to other general purpose rubbers (GPRs) of NR (natural rubber), BR (butadiene rubber), and SBR (styrene-butadiene rubber). Butyl rubbers are thus used as inner tubes, curing bladders, and curing envelope where non-stickiness is desired. The invention of bromobutyl and chlorobutyl in the 1960s, by halogenation of butyl rubbers, introduced tubeless tires, using innerliners instead of innertubes. Halogenation of the backbone double bonds on the isoprene co-monomer improved the polarity and converted the backbone double bond to allylic bromine for faster cure process. The higher polarity of BIIR (bromobutyl) and CIIR (chlorobutyl) allow their innerliner green compound sheets, prior to curing, to stick to the tire carcass for forming the innerliner layer. Improving their compatibility with GPR made it possible to use HIIR (halogenated butyl)/NR blends as innerliners and to add HIIR into tire tread compounds based on SBR, BR, and NR.
To further enhance oxidative and thermal stability of HIIR, copolymers of isobutylene and para-methyl styrene were introduced in late 1980s (European Patent No. 0344021) with no backbone or pendant unsaturations. Para-methyl styrene (PMS) was used due to its high cationic polymerizability, better than styrene, and both PMS and styrene have high cationic polymerization reactivity than that of isobutylene. High PMS incorporation in IMS (isobutylene-p-methylstyrene copolymer) became possible with PMS contents greater than 10 mole % and can reach to 90% PMS. However, the IMS component is not crosslinkable. Later, by radical bromination of the PMS moiety to generate benzyl bromide functionality, a crosslinkable BIMS (brominated IMS) was discovered (U.S. Pat. No. 5,162,445). This radical bromination process is slow and requires the use, and associated capital expenditures, of several residence tank reactors to increase the reaction time, allowing bromination process to complete. Additionally, benzyl bromides are highly reactive and specific care must be exercised to prevent compound scorching, gelling, and nerving during processing. Additionally, the bromination extent is very low due to radical bromination difficulty. Typical bromination level in a BIMS, and the resulting crosslinking level, that can be delivered economically is below 2 mole % despite the fact that one can incorporate greater than 10 mole % of PMS.
Limited unsaturations in butyls and their low polarity restrict their uses to innertubes and bladders. Halogenation of butyls leads to HIIR with higher polarity but without increasing their crosslinking sites and raising their crosslinking density. This polarity enhancement by halogenation expanded their uses with HIIR being the preferred material of choice for tire innerliners. However, there are almost no uses of HIIR in tire sidewall and tread compounds. The PIB based copolymers are prone to wear and have low elasticity, which are the limiting factors for their use in treads and in sidewalls. This propensity to wear and the low elasticity can be mitigated by raising the crosslinking density. Although one can incorporate a lot more PMS than isoprene in IMS than in butyl rubber, the PMS bromination difficulty resulting in a BIMS with still low crosslinkability while the BrPMS renders BIMS a lot more scorchy and difficult to handle, process, and store.
Accordingly, a need exists for isobutylene based copolymers that are of high polarity and crosslinkability while possessing less reactive crosslinking sites for ease of processing, scorch safety, long shelf life, and minimal Marching Mooney.